Vitamin D has many functions: Research suggests it modulates cell growth, reduces inflammation, enhances cardiovascular health, and plays important roles in neuromuscular function, immune function, and gene expression.1-3 This article focuses on the impact of vitamin D on bone health.

The term "vitamin D" can be confusing, as it fails to distinguish between the physiologically active and inert forms of the nutrient. The two major vitamin D vitamers are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). The difference between the two is the structure of their side chains, which is unrelated to their function.1 For the purposes of this article, vitamin D refers to the cholecalciferol vitamer.

This continuing education course reviews the absorption, activation, and utilization of vitamin D within the body to promote and maintain bone health, as well as the consequences of inadequate vitamin D intake throughout the life cycle. It also discusses dietary and nondietary sources of vitamin D and the current best practices surrounding testing and measurement of clinical deficiency.

Physiology of Production and Absorption
One of vitamin D's unique characteristics is that it's sourced both exogenously and endogenously. It's obtained through food and supplementation and can be produced by the body in response to adequate sun exposure.

The structure of vitamin D plays a large role in its endogenous production. Structurally, vitamin D is a secosteroid, or a steroid with a broken ring. Chemically, a steroid is an organic compound that has four rings in a specific configuration bounded by single bonds. Secosteroids have those rings in a similar configuration; however, only three of the rings are complete. The fourth ring doesn't completely link to itself but rather has an unlinked double bond that doesn't allow for the complete closure of the ring.

The vitamin D precursor (7-dehydrocholesterol) has that conjugated set of double bonds in one of its rings, which makes it a secosteroid. That broken ring allows it to absorb specific wavelengths of light. Ultraviolet B rays from the sun penetrate the epidermis and the dermis and are absorbed by 7-dehydrocholesterol in skin cells, creating precholecalciferol (or previtamin D). After a few days cholecaliciferol, also known as calcidiol,4 is formed by the reorganization of the molecule due to the heat absorbed from the ultraviolet rays. From the skin, cholecalciferol diffuses into the blood, attaches to an alpha-2 globulin vitamin D-binding protein (DBP), or transcalciferin, and is transported throughout the body.

Exogenous vitamin D (from food or supplements) is absorbed via passive diffusion into intestinal cells, primarily within the distal small intestine, although some absorption occurs in the duodenum. Since vitamin D is a fat-soluble vitamin,4 the body's ability to absorb exogenous vitamin D is dependent on adequate fat intake and proper absorption.5 Once it has diffused into the enterocyte, vitamin D is incorporated into chylomicrons and enters the lymphatic system, eventually being released into the blood. Exogenous vitamin D is delivered to the liver by chylomicron remnants and to extrahepatic tissues by DBP.

Activation
Once it reaches the liver, cholecalciferol is metabolized into the active form of vitamin D by enzymes called hydroxylases. The first, 25-hydroxylase, resides within the liver mitochondria. This enzyme hydroxylates cholecalciferol (which has reached the liver via chylomicron remnants or DBP) at the 25th carbon, forming 25-OH vitamin D (25OHD), also known as calcidiol or 25-OH cholecalciferol. Though expression of the majority of 25-hydroxylase occurs in the liver, the enzyme is distributed among the body's organs, notably the lungs, intestines, and kidneys. Once the hydroxylase enzyme has generated 25OHD, calcidiol is released into the blood as the main form of circulating vitamin D—which is currently the primary measure of vitamin D status.1,4-6 Serum concentrations of 25OHD are affected both by dietary vitamin D intake and exposure to sunlight.1,4-6

When the body senses a period of cholecalciferol deprivation, it upregulates hydroxylation activity by the 25-hydroxylase enzyme. Furthermore, when the circulating pool of calcidiol has been depleted, the body maintains vitamin D activity by releasing cholecalciferol from skin, muscle, and adipose tissue reservoirs. Measurements often are rendered in nmol/L (1 nmol/L is equal to 0.4 ng/mL). Calcidiol concentrations should be maintained above 80 nmol/L (32 ng/mL), but the ability of the body's vitamin D reservoirs to sustain normal levels of circulating calcidiol is extremely variable.4,5

Though calcidiol is the form of vitamin D used to assess vitamin D status, calcidiol isn't the physiologically active form of the vitamin. Once it's been hydroxylated in the liver, calcidiol, which is bound to DBP, is released into circulation and taken up by various tissues, most notably the kidneys. Once the calcidiol-DBP complex has been transported into renal cells by binding to megalin on the kidney's plasma membrane, a second hydroxylation of calcidiol in the kidney tubules generates 1,25(OH)2D, also known as 1,25 dihydroxycholecalciferol or calcitriol. It's this form of vitamin D that's considered the active vitamin.

The enzyme responsible for this second hydroxylation, 1-alpha-hydroxylase, is expressed in bone, intestine, skin, and macrophages but is found in highest concentrations within the kidney. Like calcidiol and 25-hydroxylase, low concentrations of calcitriol stimulate 1-alpha-hydroxylase activity, which also is influenced by parathyroid hormone (PTH) and serum calcium concentrations; high levels of PTH and calcium inhibit the enzyme, while low levels stimulate it. Dietary phosphorus also has an impact on 1-alpha-hydroxylase activity; high phosphorus intake decreases its activity (thereby decreasing serum calcitriol) while low phosphorus intake stimulates its activity to increase serum calcitriol.1,4,5,7

Mechanism of Action
Once vitamin D goes through its second hydroxylation, it's released from the kidney and transported by DBP through the blood. Calcitriol is loosely bound to DBP (unlike calcidiol, which binds tightly). With a half-life of only four to six hours, calcitriol easily releases from DBP to bind to vitamin D receptors (VDRs) on different extrahepatic tissues, including the intestines, kidney, bone, muscle, heart, pancreas, brain, and skin, as well as hematopoietic and immune system tissues.1,7

Calcitriol's interaction with intestinal VDRs makes it a critical nutrient for bone health. Calcitriol increases intestinal absorption of calcium and phosphorus via the VDR, which is a nuclear transcription factor.1,7,8 It's thought that once the vitamin interacts with cell membrane receptors in the gut to enter the intestinal cell and its nucleus, calcitriol binds to the VDR within the cell's nucleus and activates another nuclear receptor, the retinoic acid receptor (RAR). The VDR-RAR complex works with calcitriol to bind vitamin D response elements, which are small sequences of DNA. These elements promote selective DNA transcriptions along the brush border of the intestinal wall, which promote calcium absorption by a process called transcaltachia.1,7-9

The mechanism by which calcitriol increases phosphorus absorption is slightly different, as it's believed to augment alkaline phosphatase activity along the brush border.4,7 Calcitriol also may increase expression of a sodium-phosphate cotransporter in the small intestine.10

While adequate calcium absorption is crucial for bone growth and maintenance, the body tightly regulates calcium serum levels to maintain normal nervous and cardiac system function. PTH is the key player in regulating calcium homeostasis. When the parathyroid glands sense a drop in serum calcium, they secrete PTH, which then stimulates mobilization of calcium and phosphorus from bone into circulation.1,4 Increased PTH also stimulates 1-alpha-hydroxylase enzyme in the kidney, increasing levels of calcitriol and initiating a cascade of molecular interactions via VDR activation. These interactions lead to gene expressions that help normalize serum calcium through mobilization of calcium from bone, increased intestinal absorption, and increased reabsorption of calcium by the kidneys.1,4,7,11

Though PTH is primarily affected by serum calcium levels, it also could be influenced by serum calcitriol concentrations. Calcitriol may decrease the transcription of the gene for PTH, so when calcitriol levels are sufficient in the blood, genetic expression of PTH is reduced. Though the mechanism isn't completely clear, it's possible that calcitriol interacts with the VDR in the parathyroid glands, thereby influencing the regulatory region of the PTH gene.

The release of calcium from bone involves several different interactions. Either PTH or calcitriol interacts with receptors on mature osteoblasts, which induces expression of receptor activator of nuclear factor kappa-B ligand, or RANKL. RANKL then interacts with a receptor protein (RANK) on the surface of preosteoclasts to stimulate the production and maturation of osteoclasts.7 The primary function of osteoclasts is to break down bone in the remodeling process. PTH and/or calcitriol increase osteoclast activity; the osteoclasts mobilize calcium and phosphorus from bone via hydrochloric acid, collagenase, and other enzymes to dissolve the bone matrix, a destructive process called osteolysis.1,2,7,12 The outcome is the loss of bone matrix integrity to maintain homeostasis of serum calcium and phosphorus concentrations. PTH is counterregulated by the hormone calcitonin (produced in the thyroid gland), which promotes remineralization of bone by calcium and phosphorus once blood calcium begins to rise above a normal level.4,7,12

Testing Vitamin D Status
Measuring serum 25OHD is the preferred method of determining vitamin D sufficiency because it has a longer circulating half-life than 1,25(OH)2D. 25OHD circulates for about 15 days compared with 1,25(OH)2D, which has a half-life of only 15 hours. Moreover, 1,25(OH)2D levels are tightly regulated by PTH, calcium, and phosphorus, so they won't capture true vitamin D status unless vitamin D deficiency is severe.5,13,14

Assessment of vitamin D status is complicated by the existence of two different units of measurement: nanomoles per liter (nmol/L) and nanograms per milliliter (ng/mL). As stated earlier, 1 nmol/L is equal to 0.4 ng/mL.1,4,5 Lab reports may provide either unit, but the health care professional must express which unit is being reported and then interpret the results accordingly. Incorrect interpretation may lead to misdiagnoses and/or improper supplementation.

To further confound matters, there also are multiple lab assays available to test vitamin D status. The two most common methods used to measure serum calcidiol are antibody-based and liquid chromatograph-based assays. Variability also may exist among the laboratories analyzing the specimens. In summary, both the type of assay and the laboratory using that assay are variables that can lead to falsely low or high reports of vitamin D status.1,5 The National Institute of Standards and Technology released a guide in 2009 to help standardize calcidiol measurement values across different laboratories.14 In November 2010, the National Institutes of Health (NIH) Office of Dietary Supplements created the Vitamin D Standardization Program, a collaboration involving many entities and institutions dedicated to improving the detection, evaluation, and treatment of vitamin D deficiency and insufficiency.15 Vitamin D insufficiency refers to a range of serum vitamin D just below the "normal" range, while deficiency refers to a range significantly below normal. While exact cutoffs are still variable, the general progression is normal, then insufficiency, followed by deficiency. The NIH provides an excellent fact sheet that delineates the different levels of serum vitamin D adequacy, from deficiency to toxicity.

These coordinated efforts aim to promote a standardization among laboratory measurements of serum 25OHD that will improve accuracy and consistency of vitamin D measurements.16 The Vitamin D Standardization Program is ongoing, and the effects of these efforts at standardization haven't yet been determined.

Universal screening for vitamin D deficiency isn't recommended, though clinicians may find it appropriate for patients who are thought to have poor vitamin D intake or inadequate sun exposure. Other populations that would benefit from regular screening include the elderly and individuals with low bone mineral density, previous low-impact skeletal fractures (ie, stress fractures), chronic kidney disease, and digestive diseases that cause malabsorption of nutrients.17 Studies also suggest that patients with musculoskeletal symptoms such as pain or weakness would benefit from regular testing.

Populations at Risk of Deficiency
Certain diseases, conditions, and populations are associated with increased risk of vitamin D deficiency. Infants are at higher risk of deficiency because human breast milk is low in vitamin D and limited exposure to sun is common during this period of life. The elderly also are at risk due to insufficient vitamin D intake, low sunlight exposure, reduced synthesis of cholecalciferol in the skin, and impaired response of 1-alpha-hydroxylase in the kidneys to PTH. Populations with digestive conditions that impact fat absorption, such as celiac disease or inflammatory bowel disease (eg, Crohn's), also are at risk of deficiency. Impaired function of the parathyroid, liver, or kidney can diminish the ability to generate the active form of vitamin D3, leading to deficiency as well.1,4,5 Individuals on anticonvulsant drug therapy also are at risk, as they may experience an impaired response to vitamin D and/or exhibit problems with calcium metabolism.18,19 Finally, people with darker skin tones also tend to have lower vitamin D levels, but bone mass has been demonstrated to be higher in many of these populations despite lower vitamin D levels, possibly related to differences in calcium absorption efficiency, lower bone turnover rates, and other aspects of bone composition.20

Several other factors impact the skin's efficiency in synthesizing vitamin D endogenously. For instance, less vitamin D is produced at higher latitudes or during winter months, since ultraviolet B rays are longer and not as well absorbed by 7-dehydrocholesterol. The length of the ultraviolet B photon path changes based on the season of the year, time of day, altitude, latitude, and atmospheric conditions (eg, air pollution or presence of clouds). Other factors influencing risk of vitamin D deficiency include clothing (type, thickness, amount of body covered by clothing); sunscreen use; skin pigmentation; genetic variations (levels of circulating DBP); and obesity (as vitamin D is sequestered in body fat stores).21

Vitamin D Deficiency and Bone Health
Adults deficient in vitamin D may experience bone mineralization defects known as osteomalacia, or softening of the bones. This typically occurs when vitamin D deficiency reduces calcium absorption, leading to decreased serum calcium levels. This in turn stimulates release of PTH from the parathyroid. With prolonged vitamin D deficiency and the consequent diminished calcium absorption, PTH continues to promote bone resorption as well as phosphorus excretion. As calcium is brought into the blood from bone, the phosphorus that comes with the calcium from bone demineralization is excreted from the body, creating an insufficient ratio of serum calcium and phosphorus for the body to use for remineralization—thus, adequate remineralization can't occur.

As osteoclasts turn over bone during the process of bone remodeling (a process that continuously occurs throughout life), the collagenous bone matrix initially is preserved but becomes progressively demineralized. The inability to adequately mineralize bone due to an inadequate calcium-to-phosphorus ratio leads to radiographic changes that cause bone pain, osteomalacia, and increased risk of osteoporosis.1,4,7 Osteoporosis is a condition marked by decreased bone mineral density associated with increased risk of fractures.

Vitamin D deficiency in infants and children can cause rickets, a condition resulting from the failure of bone to mineralize.8 During the process of mineralization, bone-forming cells produce calcium-phosphorus crystals, which contribute to bone hardness and strength. Vitamin D deficiency prevents the body from efficiently absorbing either mineral, thereby inhibiting the formation of the calcium-phosphate crystals. As a result, bones' growth plates continue to grow without adequate mineral supply.1,4,7,22

As infants and young children grow at a rapid rate, weight-bearing limbs become bowed as bones lengthen but remain deprived of the minerals providing them with strength and hardness. Rickets is particularly dangerous in infants as it can result in skeletal deformities of the head, pelvis, legs, chest, and spine. Other symptoms include bone pain, increased bone fractures, muscle pain, and short stature.1,4,7,23 In very severe cases, vitamin D deficiency can lead to hypocalcemia, resulting in seizures. Though fortification of milk and other foods has drastically decreased the occurrence in the United States, rickets is still seen throughout the world.3,23-26

Finally, note that an indirect effect of vitamin D deficiency on bone health is its effect on muscle. Prominent symptoms of vitamin D deficiency can include muscle pain and weakness, which may increase risk of bone fractures due to falls, particularly in elderly populations, and especially those that may have osteoporosis.27,28

Vitamin D Toxicity
Though the risk of skin cancer may increase with excessive exposure to the sun, vitamin D toxicity won't occur, as the body self-regulates cutaneous production of vitamin D3.1,4 Theoretically, the sustained heat from excess sunlight exposure degrades vitamin D and its precursor as it's formed, and the conversion of 7-dehydrocholestrol into previtamin D3 will generate nonvitamin D forms that limit vitamin D3 generation.1,5

However, vitamin D toxicity from exogenous sources is possible, since 25-hydroxylase isn't well regulated.1,7 Possible toxicity is associated with levels above 375 to 500 nmol/L (150 to 200 ng/mL).5,21 Symptoms of vitamin D toxicity are nonspecific and include anorexia, weight loss, polyuria, heart arrhythmias, and hypercalcemia. Hypercalcemia eventually can lead to soft-tissue calcification, causing cardiovascular and renal damage and, in some cases, death.1,4,5 Because the dose-response relationship of vitamin D supplementation is unclear, it's difficult to narrow down what dose leads to toxicity. However, it's accepted that the 25OHD form is implicated in intoxication, and potential toxicity results when serum concentrations of 25OHD are consistently greater than 500 nmol/L (200 ng/mL).1,5,14 The threshold for vitamin D toxicity ranges from 10,000 IU/day to 40,000 IU/day.4,5,14 However, the Food and Nutrition Board at the Institute of Medicine concluded that serum 25OHD levels above 125 to 150 nmol/L (50 to 60 ng/mL) may be harmful and suggests an upper limit of 4,000 IU for individuals aged 9 and older. Comprehensive upper limits for different ages can be found through the NIH.

Vitamin D Recommendations
According to the NIH, "There is considerable discussion of the serum concentrations of 25OHD associated with deficiency (eg, rickets), adequacy for bone health, and optimal overall health, and cut points have not been developed by a scientific consensus process."4 However, Recommended Dietary Allowances are provided for vitamin D to promote adequate intake: 400 IU per day for younger than 1 year, 600 IU from 1 to 70 years of age (including pregnancy and lactation), and 800 IU for adults older than 70.4 A comprehensive chart can be found through the NIH.

Vitamin D2 vs Vitamin D3
As stated earlier, ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3) differ practically only in the structure of their side chains.1 Ergocalciferol is largely found in fortified foods and is mostly human made or plant based.20 Most food sources provide vitamin D in the form of cholecalciferol. There's no difference in their activation—both act as prohormones and reportedly display identical responses within the body. Both have been used to cure vitamin D-deficiency rickets,1,33 though there's speculation in the scientific community that vitamin D2 may be less toxic at higher doses. However, research is limited and inconclusive.1 Controversy exists in the field regarding the superiority of one form over the other. Though they both have the same practical function, cholecalciferol appears to have a longer serum half-life than does ergocalciferol. Clinically, it may be beneficial to choose the vitamin D3 vitamer over its D2 counterpart for individuals who wish to supplement with infrequent doses.16

Though evidence doesn't conclusively suggest one form as superior, some individuals prefer cholecalciferol, as it's the form synthesized naturally in the skin. Human beings can't synthesize ergocalciferol.20 Vitamin D2 can be derived from nonanimal sources (eg, fungus, yeast, or lichen), so it may be preferable for those avoiding animal products (eg, vegans).

Recommending Vitamin D to Clients
Adequate levels of vitamin D within the body are essential for maintenance of bone health throughout life. However, many aspects of vitamin D testing, diagnosis, and treatment remain controversial. With regard to dietary intake of vitamin D, health practitioners should encourage individuals to reach the minimum Recommended Dietary Allowance through dietary intake or sunlight exposure. Health professionals also should communicate the upper limits of intake to ensure excess amounts of vitamin D aren't consumed unless under the guidance of a physician to correct clinical insufficiencies or deficiencies. The Endocrine Society published evidence-based guidelines of recommended vitamin D intakes for different populations in 2011.31 This can be a valuable resource for RDs looking to determine how much vitamin D is necessary for their clients.

Because so many factors affect the skin's synthesis of vitamin D, it's difficult to determine how much an individual can synthesize at any given time. For instance, darker skin has more melanin, which makes it more difficult to absorb the sun's rays. For this reason, darker-skinned persons may need to be outside for an hour with 25% of their skin uncovered by clothing to synthesize the amount of vitamin D that lighter-skinned persons can synthesize in a few minutes. One study found that from April to October, individuals in Boston whose skin rarely burns and tans moderately can synthesize approximately 400 IU of vitamin D in three to eight minutes if 25.5% of their skin is exposed at about 12 PM. Individuals with that same skin type could spend three to six minutes in Miami and synthesize the same amount of vitamin D in any season.34 This study and others like it demonstrate that it's nearly impossible to develop standardized recommendations for sun exposure. The typical recommendation, however, is that individuals expose skin (face, arms, legs, or back) for 5 to 30 minutes (depending on the amount of melanin in their skin, which determines how quickly one burns and tans) between 10 AM and 3 PM at least twice per week.

From birth, vitamin D plays an important role in the development and maintenance of bone health in conjunction with other factors including calcium, phosphorus, and PTH. While the exact methods of vitamin D testing and assessment require further standardization, it's agreed that adequate vitamin D should be acquired either through sunlight exposure, food intake, or supplementation. It's essential for RDs to identify populations at greater risk of deficiency and recommend vitamin D testing when appropriate. If clients' vitamin D levels are insufficient or deficient, RDs can return them to optimal levels using a combination of food and sunlight exposure recommendations in addition to supplementation when needed.

— Sandeep Kaur Dhillon, MS, RDN, is a clinical dietitian at a long term care facility in New York City.

— Jason Machowsky, MS, RD, CSSD, RCEP, CSCS, is a sports dietitian and exercise physiologist at the Hospital for Special Surgery in New York City.

Learning Objectives
After completing this continuing education course, nutrition professionals should be better able to:
1. Translate how vitamin D is absorbed and activated in the body.
2. Describe vitamin D's involvement in bone metabolism and how deficiency may negatively affect bone structure throughout the lifespan.
3. Distinguish populations that are at increased risk of vitamin D deficiency.
4. Interpret the current debate and demonstrate best practices in measuring and monitoring of vitamin D levels.
5. Provide clients with several options for obtaining adequate amounts of vitamin D through food and sunlight.

CPE Monthly Examination

1. At what age do vitamin D needs increase in adults and by how much do they increase?
a. At age 50, they increase from 400 to 600 IU.
b. At age 70, they increase from 400 to 600 IU.
c. At age 50, they increase from 600 to 800 IU.
d. At age 70, they increase from 600 to 800 IU.

6. Which of the following conditions related to vitamin D deficiency is marked by reduced bone density and increased risk of fractures?
a. Osteoporosis
b. Rickets
c. Osteomalacia
d. Hypercalcemia

7. If vitamin D deficiency occurs, which of the following changes would you expect to see in a patient's calcium and parathyroid hormone (PTH) levels?
a. Serum calcium increases, and PTH increases.
b. Serum calcium decreases, and PTH increases.
c. Serum calcium increases, and PTH decreases.
d. Serum calcium decreases, and PTH decreases.

8. Which of the following populations are at risk of vitamin D deficiency due to poor dietary absorption rates?
a. Infants
b. The elderly
c. Patients with kidney disease
d. Patients with inflammatory bowel disease

9. A patient comes to your clinic presenting with a slightly deformed ribcage and bowed legs along with a history of vitamin D deficiency as a child. What condition did that patient likely have as a child?
a. Osteoporosis
b. Rickets
c. Osteomalacia
d. Hypercalcemia